The invention is directed to the hydroxylation of phenol, substituted phenols, and phenol ethers by nuclear hydroxylation of phenol with hydrogen peroxide with influence of the product selectivity in the production of ortho and para products.
Important hydroxybenzenes are derivatives of phenol, the napthols, and also derivatives of anthracene and phenanthrene. They are employed in the production of dyestuffs, in the production of synthetic resins, in photography, and for the production of important plant protectives. Thus, e.g., hydroquinone, the para hydroxylation product of phenol is used as a photo chemical; pyrocatechol, the corresponding ortho product for plant protection. For various areas of use, such as, e.g., as antioxidants, the dihydoxyphenols are mutually useful.
Their production, therefore, has long been the object of thorough investigations. The hydroxylation has been carried out both with hydrogen peroxide itself as well as with hydroperoxides, peroxides, or even per acids such as, e.g., performic acid or peracetic acid.
Nevertheless, hydrogen peroxide was preferred since it is the most readily available and since with percarboxylic acids, hydroperoxides and peroxides side reactions occur (European published application No. 0027593).
There was always present a catalyst in these hydroxylations. This-catalyst can be a metalloid such as sulfur, selenium, tellurium, phosphorus arsenic, or antimony in elemental form (German OS-No. 2348957) or there can be used boron compounds (German Pat. No. 1543830).
Various processes operate with transition elements in the form of their ions (German OS No. 2162552), especially with iron ions (German OS No. 2162589 or German Pat. No. 2407398) or cobalt ions (German AS No. 2341743), or even with corresponding oxides (Milas U.S. Pat. No. 2,395,638).
Besides, there are employed strong acids such as sulfuric acid, sulfonic acids (German OS No. 2138735, German AS No. 2410742, German AS No. 2410758, German AS No. 2462967), or a mixture of sulfuric acid and phosphoric acid (German OS No. 2138735), there are also mentioned in the last named published application organic acids such as, inter alia, trichloroacetic acid or tartaric acid.
The already mentioned percarboxylic acids likewise serve as catalysts (French Pat. No. 1479354). In all of the mentioned catalysts, it is a matter with the catalysts being solid or liquid materials. Hydrogen peroxide, as preferred oxidation agent, for the most part is employed in aqueous solutions of various concentrations up to very high concentrations which have the danger of explosion; thus, the process according to German Pat. No. 2064497 operates with solutions which only contain 5 weight % water, but even at this highly concentrated hydrogen peroxide the yield of dihydroxy derivatives was only 70% and was reduced considerably according to the dilution of the hydrogen peroxide.
Additionally, in these and also in other processes, the operation must be carried out with a very large excess of the phenol to be hydroxylated in order in general to obtain the above-stated yield. If this excess is reduced, e.g., from 20 moles to 10 moles per mole of hydrogen peroxide, then the yield is reduced drastically despite the higher concentration of hydrogen peroxide.
However, as is known, this type of excess of a reactant, which must be recycled, requires additional industrial expense; above all in regard to the size of the apparatus employed.
Since care is always taken to avoid large excesses of one component as far as possible, there have been attempts to avoid employing aqueous solutions of hydrogen peroxide.
Thus, different solutions of hydrogen peroxide in organic solvents have already been used. For example, according to the process of German Pat. No. 2410758, there are preferably employed hydrogen peroxide solutions in derivatives of phosphoric acid or phosphonic acid, namely in the presence of a strong acid, such as sulfuric acid (100%) or fluorosulfonic acid.
However, these highly concentrated strong acids have the disadvantage that their separation from the rection mixture creates difficulties (German AS No. 2658943), above all since their concentration in the reaction mixture has a considerable influence on the length of the reaction.
The excess of phenol was indeed reduced somewhat in contrast to this in the process of German AS No. 2064497, but this did not outweigh the disadvantage of the strong acids.
An additional difficulty in the process of German Pat. No. 2410758 in the working up of the reaction mixture was produced by the presence of the water formed after the reaction with hydrogen peroxide.
Since the solvent for hydrogen peroxide employed in part is higher boiling than the phenols employed and this, especially with phenol itself, frequently forms an azeotrope with water whose boiling point is below that of the organic solvent, it was highly problematic that a trouble-free separation of the excess phenols from the reaction mixture could be attained.
Therefore, other ways were tried, first to manage without catalyst, i.e., above all without the strong acids. Since the catalysts above all were needed for the activation of hydrogen peroxide, the process of German AS No. 2658943 was operated with organic solutions of peracetic acid. An additional catalyst was not used.
Entirely apart from the fact that the mentioned process presupposes a complete plant for the production of an organic percarboxylic acid, which first is obtained from hydrogen peroxide and carboxylic acid, and thereupon is produced by extraction of this so-called "equilibrium acid" from its aqueous solution, it has been shown a stated good selectivity and good yield was only possible in the presence of additional peracid stabilizers (German OS No. 2364181; European OS No. 0027593). Furthermore, this selectivity, i.e., the ratio of ortho to para products, is controlled only according to the particular hydroxylation agent, e.g., percarboxylic acids and then can be influenced only by a change of the hydroxylation agent itself, and even then only to a moderate extent (German AS No. 2658943).
Using the same hydroxylation agent, but at different reaction temperatures, there occurs practically no change in the selectivity, see Table 1 of German Pat. No. 2364181.
Also, the addition of specific, chelate complex forming materials does not produce a remedy (German Pat. No. 2364181).
Likewise, changes of the reaction time have no influence on the selectivity (European OS No. 0027593).
From what has been said above, there is no known process either in the use of hydrogen peroxide itself or in the form of its per compounds, especially its percarboxylic acids, in spite of various additives as catalysts or stabilizers, which makes possible in a specific system on the one hand satisfactory yields and on the other hand also a regulation of the ratio of ortho to pure compounds or of ortho compounds to each other, as they occur in the substituted phenols obtained in the hydroxylation. In a given system, whose essential parameters were the particular hydroxylation agent and the particular catalyst, respectively, the particular catalysts, the selectivity represents a specific factor.
Since the ortho and para compounds or the ortho compound together as isomers are not identical in their properties and, therefore, indeed in part find different industrial uses, it became desirable to be able to influence the selectivity in the production of these two isomrs without great industrial expense, i.e., above all in a still further shifting of the equilibrium in favor of one of the two isomers, especially, e.g., of pyrocatechol, or e.g., of 4-methyl-pyrocatechol. Thereby, it is essential that the predetermined parameters of a system must not be changed.